1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a facsimile machine, and a printer.
2. Description of the Related Art
A conventional electrophotographic image forming apparatus performs processes of charging, exposing, developing, transferring, and fixing in sequence, thereby printing an image. A charging section charges a photoconductive drum to a predetermined potential, and an exposing section selectively irradiates the charged surface with light to form an electrostatic latent image on the photoconductive drum. The electro static latent image is then developed with toner into a toner image. The toner image is then transferred onto a recording medium.
The toner is triboelectrically charged so that the charged toner is attracted to the electrostatic latent image, thereby developing the electrostatic latent image into the toner image. The toner image is then transferred with the aid of static electricity onto the recording medium. Therefore, the amount of charge on the toner is one of the factors that greatly affect the quality of the image printed on the print medium.
The transfer current that flows during the transfer of a toner image onto the recording medium is another factor that affects the quality of printed image. Japanese Patent Laid-Open No. H10-301344 discloses an invention in which the transfer voltage is controlled for good print quality. The invention makes use of the fact that transfer current flowing through a transfer roller greatly affects the quality of a printed image.
FIGS. 26a and 26B illustrate the relation between the print duty of a printed image and the amount of charge on the toner. A developing blade is in pressure contact with a developer bearing body or a developing roller 400 which in turn rotates in contact with a developer supplying member or a supplying roller 500. FIG. 26A illustrates the relation when an image is printed at high print duty. FIG. 26B illustrates the relation when an image is printed at low print duty. In this invention, the term “print duty” covers the amount of toner used when an image is printed on a page of recording medium, or a population density of dots printed on a page of recording medium. A developer material or toner T1 is triboelectrically charged with the aid of the friction between the developing roller 400 and the supplying roller 500. Then, the toner T1 is attracted to the electrostatic latent image, thereby developing the electrostatic latent image. When printing is performed at high print duty, residual toner T2 remains on the developing roller 400 if the toner T1 fails to be attracted to the electrostatic latent image. The amount of the toner T2 is relatively small. When printing is performed at low print duty, a relatively large amount of the residual toner T2 remains on the developing roller 400.
The toner T2 remaining on the developing roller 400 tends to be further charged due to the friction between the developing roller 400 and the supplying roller 500, becoming overcharged toner T3. The amount of overcharged toner T3 is larger when printing is performed at low print duty than when printing is performed at high print duty. As a result, the total amount of charge acquired by the toner increases gradually.
FIG. 27 illustrates the relation between the amount of toner remaining on the developing roller 400 and the amount of charge acquired by the toner, assuming that printing is performed at low print duty and only a relatively small amount of toner remains on the developing roller 400. It is assumed that the toner T1 is normally charged to a negative polarity in the present invention. Thus, an increase in the amount of charge on the toner implies that the amount of charge on the toner is large in absolute value. The toner T2 remaining on the developing roller 400 is subjected to the friction between the developing roller 400 and the supplying roller 500, being further charged to become toner T3 triboelectrically. If the amount of toner T2 remaining on the developing roller 400 is small, the amount of toner T1 supplied to the supplying roller 500 is also small. Thus, the amount of overcharged toner T3 is larger when the amount of toner T2 is smaller than when the amount of toner T2 is large. As a result, the amount of charge on the toner increases.
FIG. 28 illustrates the relation between the increase in the amount of charged on the toner and the occurrence of poor transfer performance. The toner T1 supplied to the electrostatic latent image formed on the photoconductive drum 200 is transferred onto paper P being carried on a transport belt 180. The paper P having the toner T5 thereon is transported to a fixing unit. If the amount of charge on the toner T3 is large, electric discharge occurs before the toner T5 is transferred onto the paper P, such that the toner T5 is charged to a positive polarity. The positively charged toner is not transferred onto the paper P, causing the toner to be absent from the printed image. The absence of toner is depicted at Y referred to as poor transfer performance.